The invention relates to a hydrohull which is particularly suitable for high speeds and enables the craft to be raised so that it is over the water.
It is known that the resistance encountered by a body in motion in a fluid is given by the relation: EQU R=C.sub.r 1/2rSV.sup.2
where C.sub.r is a coefficient of resistance given by the shape of the body in motion, S is its surface area, V its speed and r the density of the fluid. In a craft, the aerodynamic resistance is negligible compared to the hydrodynamic resistance so long as the craft is moving at a relatively low speed, but as speed increases the craft rises out of the water until it is planing and contact with the water becomes intermittent and occurs over a very small area. The surface area which is instead in contact with the air increases because of the lifting of the craft, so that the aerodynamic resistance rises with the increased contact area and increases at a geometric rate with the increase in speed. The overall resistance to the advance of the craft is given therefore by: EQU R.sub.0 =R.sub.1 +R.sub.2
where R.sub.1 and R.sub.2 are the hydrodynamic and aerodynamic resistances respectively. In crafts according to the state of the art, the weight of the craft is balanced by a hydrodynamic lift. Said hydrodynamic lift is proportional to the square of the speed of the craft. At the same time, also the hydrodynamic resistance is proportional to the square of the speed, so that a high hydrodynamic lift necessary to support a heavy craft necessarily generates a high hydrodynamic resistance. Moreover, also the aerodynamic resistance increases with the square of the speed and thus the power necessary to maintain the craft in motion is proportional to the cube of the speed, and a considerable amount of this power is required just to overcome the aerodynamic resistance.